Further technology may be seen in U.S. Pat. No. 3,118,979, US 2002/114478, WO 01/13678, U.S. Pat. No. 5,392,358, JP H11 178098, GB 2 311 682 and US 2007/154036.
Electrostatic sound generators are known to be superior when high frequency audio signals are desired. Electrostatic sound generators however require high voltages which hitherto have been provided using expensive amplifiers or space-consuming transformers.
Electrostatic sound generators usually operate on the basis of a high voltage difference between two back plates between which a diaphragm is fed the audio signal making, the diaphragm moves in accordance with the signal. If only a single back plate is used, the sound generator is denoted “single-sided” and the signal is then fed to one of the diaphragm and the back plate where the other is provided with a biasing charge or voltage.
Single-sided electrostatic sound generators are renowned to have a lot of second order distortion in that the force between the diaphragm and the single back plate varies with the distance between these elements. Aspects of the invention act to reduce these effects by driving the sound generator close to or at its resonance frequency, or rather providing the sound generator so as to have a resonance frequency in the operation frequency range.